The Cassegrain telescope, as invented in 1672 by Guillaume Cassegrain, consists principally of a paraboloidal primary mirror and a hyperboloidal secondary mirror positioned between the primary mirror and the primary mirror's focal point. Light impinging upon the primary mirror is reflected back to the secondary mirror. Such reflected light is again reflected by the secondary mirror to a focal plane generally located behind the primary mirror. The reflected light from secondary mirror usually reaches the focal plane by passing through an aperture in the primary mirror. At the focal plane, the operator of the telescope can use an ocular to magnify the image, project the image or photograph the image depending on the auxiliary apparatus attached to the telescope at or near the focal plane.
In order to perform planetary and lunar astrophotography, a highly specialized endeavor, it is necessary to have a high contrast image of a size suitable for photographic imagery.
As noted by J. Dragesco in High Resolution Astrophotography, Cambridge University Press, 1995, p. xiii: " . . . in high resolution work we seek to record by photography the finest details of the celestial bodies we can see with our telescopes. The resolution is defined in an angular manner, in seconds of a degree of arc(") rather than in kilometers or centimeters on the surface of the body in question." Dragesco describes, in his book, the various types of telescopes that can be used for high resolution astrophotography, particularly by amateur astronomers. An important feature of the telescope design is to have an instrument that has a long focal length (focal ratios of mirror divided by diameter of primary mirror) and a small central obstruction due to the secondary mirror and its holding structure. In the classic Cassegrain type telescope, as mentioned above, the secondary mirror is a hyperbolic mirror which is difficult to fabricate. Such difficulty in fabrication also has limited the Cassegrain design to fairly large instruments. In addition, the secondary mirror typically is 30 to 33 percent of the primary mirror's diameter. Such a large obstruction causes a degradation of the contrast of the image and alters the diffraction pattern of the image thereby reducing resolution of the telescope both of which are detrimental to high resolution astrophotography.
Various mirror combinations have been tried to overcome the impediments to high resolution astrophotography. Such combinations are described in J. B. Sidgwick's book Amateur Astronomer's Handbook, Dover Publications, Inc., 1980, pages 161-184, as well as in the Dragesco book. The typical Cassegrain telescope uses a hyperboloid secondary mirror. As explained in Sidgwick, p. 163-164, the secondary mirror should be a hyperbola so that the relation between the secondary mirror placement and the image planes remain constant. Modification to the Cassegrain arrangement, to avoid spherical aberrations, includes additional lens in the light path either at the input aperture or between the primary and secondary mirror. Further, in order to perform prime focus astrophotography, projection lenses and eyepieces have to be used with the typical classical and hybrid Cassegrain type telescopes to amplify the image at the focal plane. Several patents have issued concerning telescopes that use a Cassegrain type mirror arrangement. Kaprelian and Mimmack disclose in U.S. Pat. No. 4,061,420, highly corrected catadioptric telephoto lens that provides a long focal length in a relatively small tube length. The patent requires a corrector plate L.sub.1 as well as corrector lens, L.sub.2 and L.sub.3. The addition of such lens will affect the light passing through them. Contrast and brightness is lost in each lens medium that the light must pass through. Ames discloses in U.S. Pat. No. 5,471,346 a Cassegrain telescope that uses spherical primary and secondary mirrors. However, Ames requires corrector lens L.sub.1 and L.sub.2 to minimize aberrations resulting from such mirror combination. Again introducing additional lens elements in the light paths reduces the contrast and brightness of the object being viewed. Such loss of brightness and contrast, in astrophotography is unacceptable.